1. Field of the Invention
The invention relates to the field of quantum computing, and particularly to superconducting quantum computing.
2. Description of the Related Art
Research on what is now called quantum computing traces back to Richard Feynman. See, e.g., R. P. Feynman, Int. J. Theor. Phys. 21, 467 (1982). He noted that quantum systems are inherently difficult to simulate with classical (i.e., conventional, non-quantum) computers, but that this task could be accomplished by observing the evolution of another quantum system. In particular, solving a theory for the behavior of a quantum system commonly involves solving a differential equation related to the system""s Hamiltonian. Observing the behavior of the system provides information regarding the solutions to the equation.
Further efforts in quantum computing were initially concentrated on building the formal theory or computer science of quantum computing. Discovery of the Shor and Grover algorithms were important milestones in quantum computing. See, e.g., P. Shor, SIAM J. of Comput. 26, 1484 (1997); L. Grover, Proc. 28th STOC, 212 (ACM Press, New York, 1996); and A. Kitaev, LANL preprint quant-ph/9511026. In particular, the Shor algorithm permits a quantum computer to factorize large natural numbers efficiently. In this application, a quantum computer could render obsolete all existing xe2x80x9cpublic-keyxe2x80x9d encryption schemes. In another application, quantum computers (or even a smaller-scale device such as a quantum repeater) could enable absolutely safe communication channels where a message, in principle, cannot be intercepted without being destroyed in the process. See, e.g., H. J. Briegel et al., preprint quant-ph/9803056 and references therein: showing that fault-tolerant quantum computation is theoretically possible and opens the way for attempts at practical realizations. See, e.g., E. Knill, R. Laflamme, and W. Zurek, Science 279, 342 (1998).
Quantum computing generally involves initializing the states of N quantum bits (qubits), creating controlled entanglements among them, allowing these states to evolve, and reading out the states of the qubits after the evolution. A qubit is typically a system having two degenerate (i.e., of equal energy) quantum states, with a non-zero probability of being found in either state. Thus, N qubits can define an initial state that is a combination of 2N classical states. This initial state undergoes an evolution governed by the interactions that the qubits have among themselves and with external influences. This evolution of the states of N qubits defines a calculation or, in effect, 2N simultaneous classical calculations. Reading out the states of the qubits after evolution is complete determines the results of the calculations.
Several physical systems have been proposed for the qubits in a quantum computer. One system uses molecules having degenerate nuclear-spin states. See, e.g., N. Gershenfeld and I. Chuang, xe2x80x9cMethod and Apparatus for Quantum Information Processing,xe2x80x9d U.S. Pat. No. 5,917,322. Nuclear magnetic resonance (NMR) techniques can read the spin states. These systems have successfully implemented a search algorithm, see, e.g., M. Mosca, R. H. Hansen, and J. A. Jones, xe2x80x9cImplementation of a quantum search algorithm on a quantum computer,xe2x80x9d Nature 393, 344 (1998) and references therein, and a number-ordering algorithm, see, e.g., L. M. K. Vandersypen, M. Steffen, G. Breyta, C. S. Yannoni, R. Cleve, and I. L. Chuang, xe2x80x9cExperimental realization of order-finding with a quantum computer,xe2x80x9d preprint quant-ph/0007017 and references therein. (The number-ordering algorithm is related to the quantum Fourier transform, an essential element of both Shor""s factoring algorithm and Grover""s algorithm for searching unsorted databases.) However, expanding such systems to a commercially useful number of qubits is difficult. More generally, many of the current proposals will not scale up from a few qubits to the 102xcx9c103 qubits needed for most practical calculations.
Further, current methods for entangling qubits are susceptible to loss of coherence. Entanglement of quantum states of qubits can be an important step in the application of quantum algorithms. See for example, P. Shor, SIAM J. of Comput., 26:5, 1484-1509 (1997). Current methods for entangling phase qubits require the interaction of the flux in each of the qubits, see Yuriy Makhlin, Gerd Schon, Alexandre Shnirman, xe2x80x9cQuantum state engineering with Josephson-junction devices,xe2x80x9d LANL preprint, cond-mat/0011269 (November 2000). This form of entanglement is sensitive to the qubit coupling with surrounding fields, which cause decoherence and loss of information.
Currently proposed methods for readout, initialization, and entanglement of a qubit involve detection or manipulation of magnetic fields at the location of the qubit, which make these methods susceptible to decoherence and limits the overall scalability of the resulting quantum computing device. Thus, there is a need for an efficient quantum register where decoherence and other sources of noise are minimized but where scalability is maximized.
In one embodiment, a two-junction phase qubit includes a superconducting loop and two Josephson junctions separated by a mesoscopic island on one side and a bulk loop on another side. The material forming the superconducting loop is a superconducting material with an order parameter that violates time reversal symmetry.
In one embodiment, a method is provided for controlling an information state of a qubit having a superconducting loop that includes a bulk loop portion, a mesoscopic island portion, and two Josephson junctions separating the bulk loop portion from the mesoscopic island portion. The method includes applying a bias across the mesoscopic island portion. In one embodiment, the method includes driving a bias current in the superconducting loop. In one embodiment, the method includes driving a bias current in the superconducting loop by coupling a magnetic flux into the superconducting loop.
In one embodiment, a qubit apparatus includes a superconducting loop having a bulk loop portion, a mesoscopic island portion, and two Josephson junctions separating the bulk loop portion from the mesoscopic island portion. A control system is provided for applying a bias current across the mesoscopic island portion. In one embodiment, the control system includes a tank circuit inductively coupled to the superconducting loop.
In one embodiment, a method for fabricating a Josephson junction includes, depositing a relatively thin film of superconducting material, depositing an amorphous carbon layer onto the superconducting material, etching a qubit pattern onto a chromium mask, transferring the qubit pattern to the amorphous carbon layer by patterning through the chromium mask, etching the pattern into the superconducting material, and removing the amorphous carbon layer from the superconducting layer. In one embodiment, the superconducting layer is deposited by pulsed laser deposition. In one embodiment, one or more layers of gold are deposited onto the superconducting layer. In one embodiment the amorphous carbon layer is deposited by e-beam evaporation. In one embodiment, the etching includes argon ion milling.
In one embodiment, a two-junction phase qubit includes a loop of superconducting material, the loop having a bulk portion and a mesoscopic island portion. The loop further includes a relatively small gap located in the bulk portion. The loop further includes a first Josephson junction and a second Josephson junction separating the bulk portion from the mesoscopic island portion. The superconducting material on at least one side of the first and second Josephson junctions has an order parameter having a non-zero angular momentum in its pairing symmetry.
In one embodiment, a qubit includes a superconducting loop having a bulk loop portion and a mesoscopic island portion. The superconducting loop further includes first and second Josephson junctions separating the bulk loop portion from the mesoscopic island portion. The superconducting loop further includes a third Josephson junction in the bulk loop portion. In one embodiment, the third Josephson junction has a Josephson energy relatively larger than a Josephson energy of the first and second Josephson junctions.